There is known, for example, an inkjet recording apparatus as an image forming apparatus such as a printer, a facsimile or a copy machine apparatus. An inkjet recording apparatus performs recording by discharging ink droplets from a recording head onto a recording medium such as a recording paper (hereinafter, simply referred to as a “paper” but material is not limited to a paper). The inkjet recording apparatus is capable of recording a fine image at a high speed with advantages such as a low running cost, a low noise and an easy color image recording using multi-color ink.
In such an inkjet recording apparatus, it is required to increase a positional accuracy of landing positions of ink droplets on a paper so as to improve an image quality. There is known, such as disclosed in Japanese Laid-Open Patent Applications No. 4-201469, No. 9-254460 and No. 2000-25249, an inkjet recording apparatus that uniformly charges a conveyance belt to attract a paper by an electrostatic force to maintain a distance between a recording head and the paper constant, and to prevent an offset in a position of the paper by accurately controlling a paper feed, and to prevent a lift of the paper so as to prevent jamming and contamination of the paper due to a contact between the paper and the recording head.
However, it is known that when a conveyance belt is uniformly charged at a positive voltage to attract a paper by an electrostatic attraction force, ink droplets injected from a recording head are influenced by an electric field, which causes offsets in landing positions of the ink droplets on the paper and a reverse flow of ink mist toward the recording head.
In order to prevent offsets in landing positions of ink droplets and reverse flow of ink mist, there is known a charging method such as disclosed in Japanese Laid-Open Patent Application No. 2000-25249, in which ink droplets being injected is prevented from being influenced by an electric field by weakening a potential of a surface of a paper by applying an electric charge having a polarity opposite to a charge of a conveyance belt, of which surface is uniformly charged, on an upstream side of a recording head in the conveyance direction. Additionally, the paper is cause to be attracted by the conveyance belt by an electrostatic attraction force by weakening a potential of the surface of the paper having the same polarity as the surface of the conveyance belt.
Further, as a charging method of a conveyance belt, there is known a method such as disclosed in Japanese Patent No. 2897960, in which an alternating charge pattern is formed on the surface of the conveyance belt by applying positive and negative charges alternately onto the surface of the conveyance belt by causing a voltage applying means being brought in contact with the surface of the conveyance belt.
When attracting and retaining a paper by an electrostatic attraction force as mentioned above, an electric field is generated between a surface of the paper and the recording head and ink droplets discharged from a recording head are polarized due to an influence of the electric field. Thus, there is a problem in that a good recording cannot be achieved due to a turbulent flow and also ink mist generated by the flow of ink droplets may flow reversely and adhere to a head discharge part.
With respect to this problem, as disclosed in Japanese Patent No. 2897960, it was found that the electric field that is a cause of the offsets of landing positions of ink droplets and the reverse flow of ink mist can be weakened by applying alternating charges (positive and negative charges by an alternating current) onto a conveyance belt so as to generate an attraction force between the paper and the conveyance belt and simultaneously canceling the positive and negative charges by exchanging the positive and negative charges induced on the surface of the paper to reduce a potential of the surface of the paper.
That is, in order to weaken the electric field, a time period (hereinafter, referred to as a charge period length) of one cycle of an area where positive and negative charges are alternately applied may be reduced, that is, the charge period length formed by a pair of positive and negative charges may be shortened. This is because if the charge period length is shortened, a distance for the positive and negative electric charges to travel is reduced, which reduces a resistance giving an influence to the travel of the electric charges.
However, if a conveying speed is increased while reducing a charge width to weaken the electric field so as to aim at coexistence with improvement in the printing speed, which has become important in recent years, there is generated fluctuation of discharging area in a corona discharge area due to banding of a charge roller for charging the conveyance belt. Accordingly, electric charge cannot be applied stably and charge fluctuation occurs, which results in fluctuation in an attraction force necessary for conveying the paper, which consequently causes conveyance failure.
Additionally, there are problems in that: a sufficient charge cannot be applied to a conveyance belt since there is a limitation in the response speed of the output of a high voltage power supply; a failure may occur in the high-voltage power source if the conveyance speed is increased forcibly; and pin holes may be formed in the conveyance belt due to insulation destruction caused by a local high-voltage being generated.